Designing string tokenisers

#1

Char *mystrtok(char str,char delim)
{
static charinput = NULL;
if(str!=NULL)
{
input=str;
}

  if(input==NULL)
  {
        return NULL;
   }

  char *output=new char[strlen(input)+1];
  int i= 0;
  for( ; input[I]!='\0';i++)
  {
       if(input[i] != delim)
       {
            output[I] = input[i];
        }
       else
       {
            output[i] ='\0';
            input=input+i+1;
            return output;
    }
}
//Corner case
output [i] = '\0' ;
input =NULL;
return output;

}

my question is why here sir used if(input==NULL)
{
return NULL;
}

In for loop sir already made this condition that
input[I] != ‘\0’

#2

this condition will be executed at end when all tokens are extracted
and nothing is left then your input ==NULL
at that time you also have to return NULL

to handle this last case sir have made this condition

#3

But in corner case this condition is covered .And in corner case Output[i] is also asigned to ‘\0’ which acts as null .

#4

if input is null then this will not throw an error
strlen(NULL);